1. Field of the Invention
This invention relates to pre-distortion circuits for improving the linearity performance of precision analog signal processing circuits such as electro-optical apparata, radio-frequency (RF) transceivers and Analog-to-Digital (A-to-D) converters; and more specifically, to amplitude-stabilized pre-distortion circuits.
2. Discussion of the Related Art
The performance of A-to-D conversion chains (including signal conditioning front-ends, automatic variable gain adjustment VGA, the ADC proper, and other ancillary circuits) is described by a number of electrical parameters, among which a paramount importance is given to distortion. The distortion characteristics of the circuitry are rated in terms of SFDR (Spurious-Free Dynamic Range) or also, and especially in the case of static inputs, by the INL (Integral Non-Linearity). The main harmonic distortion contributors in conversion front-ends apparata are usually the second and third order distortion tone, that can be caused by input non-linear capacitance, non-linear resistance, and more traditional sample/hold and quantizing stage non-linearities such as gain compression. It can be mathematically demonstrated that the gain variation against the output amplitude of an opamp such as 102 shown in FIG. 1, closed in a Sample/Hold feedback loop such as the classic “flip-around” configuration 104 leads to a second-order tone, or a parabolic shape of the INL of the ADC chain.
There are instances (such as in the case of non-linear capacitance) when the input signal frequency plays a major role in dictating the shape of the INL error; in the case of opamp gain modulation, the INL shape tends instead to remain constant against either the input signal frequency, or the clock (sampling signal) frequency. The compensation of the first kind of distortion generally requires a localized circuit solution that counters any C or R modulation by utilizing opposite variations of elements of the same electrical nature. However, INL errors of the second kind, invariant with regards to frequency (and supply, and temperature, and pressure changes, to the largest extent) can be compensated by an ad-hoc circuit, not necessarily related in kind to the cause of the distortion: in fact, the INL distortion can be thought of as a target of the compensation process, regardless of the specific A-to-D conversion block causing the distortion, provided it does not vary substantially with the aforementioned physical variables. Otherwise, some form of “compensation tracking” of the INL error can be devised to minimize the final INL error after compensation in all possible conditions, but—the compensation circuit being unrelated to the original cause of the error—the tracking will most always be approximate, and complicate to a large degree the electrical circuit solution to the original problem.
An additional source of signal harmonic distortion, as added to the parabolic modulation e.g. of an opamp's gain as detailed in a related disclosure, is the so-called “INL S-shape” or third-order distortion shown in FIG. 2. This common signature found in an A-to-D conversion system 202 non-linearity can be generated, e.g., by an intermodulation of the input signal with the voltage reference against which the signal is supposed to be weighted, or by the resistive non-linearity of any MOSFET switches found in a signal path. An all-analog method to correct for the third order distortion generated by this and other mechanisms is desirable. A conceptual correction path 204 and third-order compensation signal is also shown in FIG. 2.
What is desired, therefore, is a pre-distortion circuit that will substantially compensate for second-order and more in general even-order errors, as well as third-order and more in general odd-order errors, for example in an A-to-D conversion chain; and will do so over all process, voltage, and temperature corners, and in presence of radiation.